St. Andrews Channel is a major, deep-water arm of the expansive Bras d'Or Lake system on Cape Breton Island. This is a brackish inland sea, where saltwater from the Atlantic Ocean mixes with freshwater from several rivers, creating a unique marine ecosystem. The channel is notably deep, with a maximum depth reaching 287 meters (942 feet) along its central axis, forming a significant underwater basin. It connects to the Atlantic via the narrow, winding Little Bras d'Or Channel to the northeast and is part of a network that includes East Bay and Great Bras d'Or Channel. The water salinity varies but is generally lower than the open ocean, and the restricted connections to the sea result in a reduced tidal range. The surrounding landscape features steep, forested hills rising from the shoreline, contributing to its dramatic, fjord-like character.